The LightSail 2, a crowdfunded solar sail designed by Bill Nye’s nonprofit organization, The Planetary Society, recently flew through space powered solely by sunlight. In technical terms, the craft increased its orbital high point by 2 kilometers (1.2 miles), thereby demonstrating that solar sailing is possible.
This is a remarkable achievement in its own right, but it could have even more profound effects on the future of space travel.
How solar sailing works
Solar sailing works by using a shiny sail that reflects particles of light, called photons. The photons bounce off the sail and provide a tiny amount of momentum, pushing the craft forward like the wind blowing into a ship’s sails. Although the generated momentum is very small, because there is no air resistance in space, this method can accelerate ships to potentially much higher speeds than traditional propulsion.
Putting satellites into orbit
The first application of solar sailing will be placing small satellites like CubeSats into orbit. Demand for satellite launches continues to grow, especially in the communications industry, and solar sailing provides an efficient way to place these satellites.
The LightSail 2 is a tiny craft, just about the size of a loaf of bread, but is similar to the CubeSat size of 10 x 10 x 10 cm. Solar sailing could be used to adjust the orbit of a satellite or to “hover” it over a particular location. Typically, a satellite would be pulled by the gravity of the sun, but the solar sail could provide propulsion to offset this, keeping the satellite fixed in one location.
The primary objective of the LightSail mission was to demonstrate the feasibility or placing satellites, LightSail program manager and Planetary Society chief scientist Bruce Betts said in a statement: “Our criteria was to demonstrate controlled solar sailing in a CubeSat by changing the spacecraft’s orbit using only the light pressure of the sun, something that’s never been done before. I’m enormously proud of this team. It’s been a long road and we did it.”
Visiting other planets
The future of solar sailing could encompass more than just satellites, however. The same technology could be used to help us visit other planets within our solar system.
As sunlight is stronger closer to the center of the solar system, solar sailing would be most effective for visiting the inner planets of Mars, Venus, and Mercury. It would take months or even years for a solar sailing ship to reach the inner planets, but it could be used to send supplies to Mars to support manned missions, for example. Rocket launches are extremely expensive, not to mention environmentally damaging, so a solar sail provides an alternative for future resupply missions.
A craft powered by solar sails would still need a rocket to launch it, but it only needs to be taken into low-Earth orbit, which requires much less fuel. Once in orbit, the craft could unfurl its solar sail and use sunlight to propel itself to its destination. In his book Space Sailing, World Space Foundation co-founder Jerome Wright estimated that using solar sails rather than rocket launches could save more than $10 billion in mission costs.
The European Space Agency has proposed using solar sailing to visit Mercury, which is extremely difficult to reach using rockets due to its proximity to the sun. A solar sail craft could not only visit the planet, but also return samples from it to Earth.
With some patience, the same principles could be used to visit the outer planets like Jupiter and Saturn, which could take two years or more. The challenge with this is that the craft would be traveling very fast, so it would need to be slowed down somehow when it reached its destination.
Aerospace engineer Robert Zubrin has proposed that we could effectively terraform Mars by placing a stationary satellite over the planet’s pole. The satellite could reflect sunlight onto the polar ice cap and thereby warm the planet’s atmosphere, making it more hospitable to humans.
To achieve this, the satellite would need to stay in one location in a way that is not possible using geosynchronous orbit. However, solar sails would allow the satellite to become a “statite” that hovers in place in the same way that a CubeSat could hover over Earth.
Visiting other solar systems
It may even be possible to use this technology to visit other stars. To achieve this in a reasonable time frame, we would need to find a way to boost the power going to the sails to a high enough level that a craft could travel much faster than is currently possible.
This is where we reach science fiction territory. Physicist and sci-fi author Robert Forward proposed that one way to do this would be to install high energy lasers powered by sunlight that orbit around the sun. These lasers could be trained toward a solar sail ship, and as lasers have high energy, they would propel the craft to high speeds and send it shooting off out of thesolar system.
The problem with this method, once again, is how to slow the craft once it reaches its destination. Forward proposed a double sail system in which part of the sail is cut free as it approaches its target, reflecting the beams back onto the main part of the sail and slowing the craft down.
Beyond solar sailing: Microwave sails
An alternative to powering a craft by visible light would be to use microwaves, which can be sent further out into space and therefore propel a craft further than even lasers. Forward also came up with the concept of the Starwisp probe that has a microwave sail which uses microwave beams to accelerate by up to 24 meters per second, allowing the craft to reach a significant fraction of the speed of light before it passed beyond the reach of the microwave emitters.
Such probes would still take decades to reach other stars, but they would be relatively cheap to produce and launch. Hundreds or even thousands of these probes could be sent out to investigate distant parts of the galaxy, gathering information as they travel.
Next steps for solar sailing
Coming back to the realm of reality, the next project to show the uses of solar sailing will be NASA’s Near-Earth Asteroid Scout, a CubeSat powered by solar sails. The uncrewed reconnaissance mission will target a yet-to-be-selected asteroid in our solar system and could launch as soon as next year.